1. Field of the Invention
The invention relates to a quasi-optical gyrotron in which an electron-beam gun with an annular cathode generates an electron beam, which passes along an electron beam axis and in so doing is compressed by a static magnetic field and forced into gyration, so that it excites in a quasi-optical resonator, which exhibits two mirrors arranged opposite to one another on a resonator axis aligned perpendicular to the electron beam axis, a standing alternating electromagnetic field of specific wavelength.
2. Discussion of Background
A quasi-optical gyrotron of the type initially mentioned is known for example, from the Patent CH-664045 or from the article "Das Gyrotron, Schlusselkomponente fur Hochleistungs-Mikrowellensender" (The gyrotron, key component for high-power microwave transmitters), H. G. Mathews, Minh Quang Tran, Brown Boveri Review 6-1987, pages 303 to 307. Such a gyrotron operates at frequencies of typically 150 GHz and above and is capable of generating radiant powers of several hundred kilowatts in continuous-wave operation.
For electro-optical reasons, a gyrotron of the type mentioned has an annular electron beam. When this electron beam enters the resonator, a certain portion of the current passes through nodal surfaces of the standing alternating electromagnetic field, and thus contributes only insubstantially to the excitation of this field. On the other hand, at the antinodes of the standing alternating field, the azimuthal kinetic energy of the electron beam is largely converted into microwave energy. It is thus unavoidable that a certain proportion of the energy of the electron beam cannot be used.
This problem has already been recognized, and appropriate proposals for a solution have, moreover, already been made. In the publication "The NRL Quasi-optical Gyrotron Experiment", T. A. Hargreaves et al., Twelfth International Conference on Infrared and Millimeter Waves, Dec. 14-18, 1987, Lake Buena Vista (Orlando), Fla., Conference Digest by R. J. Temkin, pages 239 to 239, it is proposed to use an electron gun with a sheet beam to improve the efficiency, instead of a magnetron injection gun.
A concrete proposal for a sheet-beam gun is known, however, from the publication "Design of a Quasi-optical Gyrotron with Sheet Electron Beam", M.E. Read et al., Thirteenth International Conference on Infrared and Millimeter Waves, Dec. 5-9, 1988, Honolulu, Hawaii, Conference Digest by R. J. Temkin, pages 279 to 280. In this instance, several, strip-shaped cathode elements, which emit electrons and deliver the desired sheet beam, are arranged parallel to one another.
Although such an electron beam is capable of substantially enhancing the efficiency of a gyrotron, it is expensive to produce and requires a completely novel electron beam optical system. That is to say, the gyrotrons built to date are designed for a rotational symmetrical beam and cannot therefore directly be operated with the novel sheet-beam gun.
In aiming further to enhance the efficiency of the quasi-optical gyrotron, it is also necessary to be able to increase the current density of the electron beam. A significant step in this direction was made with the development of so-called mixed-metal matrix cathodes, such as are known, for example, from Patent EP 0,157,634 B1. The high current densities (above 10 A/cm.sup.2) that mixed-metal matrix cathodes are capable of producing, and the compatibility with the conventional electron beam optical system enable an improvement of the total efficiency of the known gyrotron structure.
In this connection, reference is also to be made to the technology of impregnated cathodes and of so-called dispenser cathodes. An overview of this technology is given by the article "Dispenser Cathodes; The Current State of Technology", L. R. Falce, Hughes Aircraft Company, Electron Dynamics Division, Torrance, Calif., IEDM 83, pages 448 to 451, IEEE 1983. Finally, it is known from the article "Performance Analysis of three different M-Type Dispenser Cathodes", B. Latini, P. Cristini, I. Fragela and G. Marletta, Int. Conf. on Microwave Tubes in Systems, Problems and Prospects, London, Oct. 22-24, l984, Conf. Publ. No. 241, pages 35 to 41, that the current density of the cathodes can be substantially enhanced by coating with suitable metals such as, for example, Os/Ru, Os/W, Ir and the like.